Rename t8 cmesh vtk

[Davknapp]

With the upcoming PRs I will add a vtk-reader. To distinguish between reader and writer I renamed the file t8_cmesh_vtk to
t8_cmesh_vtk_writer.
In the upcoming PRs, I will add a vtk-reader.

All these boxes must be checked by the reviewers before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

• The author added a BSD statement to doc/ (or already has one)

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

• All tests pass (in various configurations, this should be executed automatically in a github action)

• New source/header files are properly added to the Makefiles

• New Datatypes are added to t8indent_custom_datatypes.txt

• The reviewer executed the new code features at least once and checked the results manually

• The code is covered in an existing or new test case

• New tests use the Google Test framework

• The code follows the t8code coding guidelines

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.